Plies of the type mentioned are already known from the prior art. Such plies are used as separators in batteries and capacitors in energy storage duty. Charge storage in batteries and capacitors takes place chemically, physically or in a mixed form, for example by chemisorption.
To avoid an internal discharge within the battery or capacitor, oppositely charged electrodes are separated from each other mechanically by means of materials which do not conduct electrons and are known as separators or spacers. At the same time, by virtue of their porosity being conformed to the energy storage system and its use, the separators or spacers make it possible for ionic charge-carriers of an electrolyte to move between the electrodes.
The separators known from the prior art have small, interlinked openings in the micrometer range. These openings are said to be as large as possible in order that electrolyte conductivity in the drenched separator be as high as possible and the battery thus have a high power density. However, if the openings are too large, then metal dendrites can lead to a short circuit between the two electrodes which are actually to be electrically separated from each other. The metal dendrites consist either of lithium or of other metals which can be present in the battery as impurities.
Furthermore, particles of electrically conductive electrode materials can migrate through the openings. These processes can give rise to a short circuit between the electrodes and greatly speed the self-discharging of the battery or capacitor.
A short circuit can result in the local flow of very high currents, which releases heat. This heat can cause the separator to melt, which in turn can lead to a distinct decrease in the insulating/isolating effect of the separator. A very rapidly self-discharging battery consequently constitutes a high safety risk because of its high energy content and also the combustibility of the electrolyte and of other constituents.
A further disadvantage with separators known from the prior art is their lack of stability in the event of rising temperatures. The melting point is around 130° C. when polyethylene is used and around 150° C. when polypropylene is used.
Causes of short circuits include shrinkage of the separator due to excessive high temperature in the battery, metal dendrite growth due to reduction of metal ions (lithium, iron, manganese or other metallic impurities), debris from electrode particles, cutting debris or broken covering on electrodes, and direct contact between the two flat electrodes under pressure.
EP 0 892 448 A2 discloses the shutdown mechanism. The shutdown mechanism responds to local heating, for example due to a short circuit, by counteracting the aerial spreading of the short circuit by prohibiting ion migration in the vicinity of the initial short circuit. The heat loss due to the short circuit causes polyethylene to heat up to such an extent that it will melt and blind the pores of the separator. Polypropylene, which has a higher melting point, stays mechanically intact.
US 2002/0168569 A1 describes the construction of a separator consisting of polyvinyl difluoride which, in the manufacturing operation, is incipiently solubilized with a solvent, mixed with silica particles and applied as a thin film. Removing the solvent leaves a porous membrane.
WO 2006/068428 A1 describes the production of separators for lithium ion batteries by using a polyolefin separator which is additionally filled with gellike polymers and inorganic particles.
WO 2004/021475 A1 describes the use of ceramic particles which are combined with organosilicon adhesion promoters and inorganic binders from oxides of the elements silicon, aluminum and/or zirconium to form a thin sheet material.
To achieve adequate mechanical flexibility, the ceramic particles are incorporated into a supporting material, for example a fibrous nonwoven web fabric. This is disclosed by WO 2005/038959 A1.
To prevent short circuits in the initial stages of metal dendrite formation, WO 2005/104269 A1 describes the use of comparatively low-melting waxes as an admixture to a ceramic paste.
WO 2007/028662 A1 describes the addition of polymer particles having a melting point of above 100° C. to ceramic fillers in order that the mechanical properties of the separator may be improved. The materials described are intended for use as a separator for lithium ion materials. Although these separators do provide a higher thermal stability than membranes, they have so far not been a commercial success. This may be due to their relatively high costs and to the excessive thickness of the material, which is above 25 μm.
WO 2000/024075 A1 describes the production of a membrane which can be used in fuel cells. This membrane consists of glass fiber materials in which fluorinated hydrocarbon polymers are fixed by means of a silicate binder.
Finally, JP 2005268096 A describes a separator for lithium ion batteries which is produced by melting together thermoplastic particles in a polyethylene/polypropylene fibrous supporting material by heating. This separator has a bubble-shaped porous structure having a pore diameter of 0.1-15 μm.
The prior art does not show an inexpensive separator which combines low thickness with high porosity and high thermal stability and can be safely used, over a wide temperature range, in batteries having high power and energy density.